This invention relates to a brake control apparatus and particularly to a proportional control circuit for a permanent magnet brake of the type wherein a permanent magnet creates a magnetic flux path for applying braking torque and an electromagnet provides a continuously variable magnetic flux in opposition to the permanent magnet. This invention has particular application to wheeled vehicles such as materials handling vehicles.
The brakes of many materials handling vehicles are spring applied and are released by the application of either hydraulic pressure or by means of an electric solenoid in opposition to the force of the spring. In those brakes, the amount of braking torque is controlled by the spring force applied to the brake shoes. One example of a control system for a brake of this type is shown in U.S. Pat. No. 5,205,620 where a materials handling vehicle, a turret stockpicker, includes brakes on the pair of load wheels which are adjusted to have one level of braking torque, while brakes on the pair of steered wheels are adjusted to have a second, usually lower, level of braking torque. Either pair of brakes may be applied individually or all together to provide three different levels of braking according to the vehicle speed.
Another brake is shown in U.S. Pat. No. 5,057,728. Here, the brake element itself includes two sets of springs and wherein a pair of electromagnets control which spring sets are applied at any given time thereby to provide a three step control over braking torque.
Proportional control of braking torque for spring actuated brakes is difficult because of the variable gap between the brake armature and the solenoid or electromagnetic coil acting on the armature, and also the variable forces applied by the spring to the armature. It would be desirable, however, to provide a proportional and continuous control of braking torque on a material handling vehicle.